There are situations in which it is necessary to inject or feed one liquid into the stream of another liquid. Some liquid pumping systems require an occasional injection of liquid while others need a more continuous feed of the liquid. Still others might require a combination of the two. For purposes of this disclosure, it is understood that the term “feed” will include inject.
One such common application is in the field of water treatment wherein certain chemicals, such as chlorinating solutions, fluorination chemicals and other liquids, are fed into the water stream at a point prior to its delivery for end use by consumers. It is important to maintain certain percentage levels of these added liquids in order to assure adequate functionality without exceeding predetermined concentrations which could be objectionable or even harmful to the consumer.
A variety of apparatus is available in the industry to perform this chemical feed task. Such apparatus typically takes the form of a pump, wherein pump speed and chemical feed rate is controlled by well known electronic means which employs chemical concentration detection means and provides voltage or current signal output for use by the pump drive system to adjust its feed rate. This system operates in a closed loop fashion to maintain a relatively stable concentration of the desired chemical in the water stream.
Certain chemicals, particularly sodium hypochlorite (NaOCl) solution used for chlorination of the water system, exhibit the troublesome characteristic of constant gas generation. Specifically, the liquid NaOCl spontaneously outgases in such a way that bubbles form in conduit piping, fittings and any other cavities in the feed circuit. Positive displacement pumps attempting to draw this liquid from storage tanks and feed it into the water stream can become gas-bound when encountering such gas bubbles. Once gas-bound, the pump will simply work against a “springy” bubble, which will alternately compress and expand to entirely devour the pump's displacement stroke volume. At this point, feeding of liquid chemical into the water stream ceases and the pump will uselessly run without effect.
This problem is aggravated by the often encountered requirement to feed the liquid chemical directly into a pressurized water stream. Here, even a modest sized gas bubble will give rise to a gas bound condition as the pump unsuccessfully attempts to compress the gas sufficiently to force it out of the pump chamber against the water stream back pressure. The problem is sufficiently severe that certain water treatment facilities undertake the extra step of diluting the sodium hypochlorite solution in the liquid chemical supply tank in order to reduce gas bubble formation. It can be reliably stated that the most aggravating problem known in the water chlorination and disinfection industry is the off-gas generated by the sodium hypochlorite NaOCl solution.
Another related problem is associated with priming. Once a chemical vessel is emptied, the feed apparatus will draw in air and entirely fill the intake circuit (including tubing, fittings, internal chambers and such) with this air. The chemical concentration detection apparatus will then signal or alarm for intervention by a technician. Chemical feed restoration now requires that a full liquid chemical vessel be substituted for the empty vessel followed by a troublesome and time consuming sequence of valve openings/closings by a skilled technician to bleed offending air out of the circuit in order to prime the pump. Only after the technician confirms by observation that the feed pump is actually feeding liquid into the water stream can the task be considered completed. This problem of manual bleeding is common to any liquid chemical application and is in addition to and apart from the out-gassing characteristics of NaOCl solutions.
Numerous attempts have been made to solve the problems described herein. For example, it is known in the field to incorporate a solenoid operated purge valve in a liquid pump, which is manually or automatically operated to divert the pressure output port of the feed pump away from the pressurized water stream and back to the liquid chemical supply tank. Once liquid has filled the pump circuit, the valve is shifted back so as to direct the chemical liquid into the pressurized water stream. However, the drawbacks of such prior art solutions include complex electronics, additional valves, manual intervention or urgent attention on the part of technicians.
Another problem associated with liquid NaOCl pumping systems is the corrosive effect that chlorine vapors have on the various metal components of the system. Specifically, metal screws, clamps, and even stainless steel components are vulnerable to corrosion by exposure to these chlorine vapors.
Still another design consideration with such pumping systems is the necessity that the system only pump in one direction. For example, in the field of water treatment, wherein a chlorinating solution is fed into a water stream, it would be very detrimental if the pumping system were to malfunction and pump in the reverse direction whereby water from the water stream is pumped into a liquid NaOCl supply. Since most motors used in such pumping applications are typically direct current (DC) motors, such malfunction could occur, for example, if the polarity of the current flowing to the motor were somehow reversed. Reversed pumping can also occur if the pumps and motor couplings are not properly oriented with respect to the motor upon installation.
The pumps themselves must also be carefully designed to prevent any leakage. Proper operation of these pumps is largely dependent on the precise angular and axial orientation of the pump piston with respect to the pump's inlet and outlet ports. Any misalignment between the two can result in a pressure build-up within the pump causing the end cap of the pump housing to rupture.
Another difficulty encountered in certain pumping applications of this type has been associated with loss of supply liquid. This can occur if the pumping unit is installed in a remote location with little or no routine maintenance, combined with no monitoring of supply vessel liquid level. In such situations, loss of supply liquid to the pumping unit will result in termination of “hypo” injection into the water stream. This loss of supply liquid will be detected by monitoring equipment, which normally controls the pumping injection rate by speeding up the pump motor or slowing it down accordingly when free supply liquid percentages fall or rise.
The loss of supply liquid will, if not corrected, lead the detection equipment to attempt to raise the supply liquid levels by directing the motor of the pumping unit to increase its speed. This speed will increase all the way to maximum, where it will remain until a technician intervenes. Such intervention may not occur for hours or days. Meanwhile the pump will be running at high speed with no liquid to cool or lubricate its moving parts. It has been found under such circumstances that the pump components will heat up from friction effects to the point where drag gradually increases and the pump eventually seizes. This can sometimes cause the drive motor to burn out if the drive electronics is not adequately fused or the pump elements can become fused together such that full dismantling is required in order to free them.
Accordingly, it is desirable to provide a simply designed system, wherein gas bubbles are dispatched automatically while replacement of an empty liquid chemical supply tank and commissioning of a new full tank is simply done by switching input tubing from the empty to the full tank. It would be further desirable to provide an apparatus requiring no priming and does not require the pump to be turned off when changing liquid supplies. It would also be desirable for such an apparatus to include pumps that are substantially leak-free, rupture-free and less prone to chemical precipitate build-up with resultant mechanical failure. It would still be further desirable to provide a liquid NaOCl pumping system that is less vulnerable to the corrosive effects that chlorine vapors have on the various metal components of the system and that is safe-guarded for pumping in only one direction. It would also be desirable to provide a liquid pumping system that will not be damaged in the event of a loss of supply liquid.